This is a PHP implementation of the Money pattern, as described in [Fowler2002] :

A large proportion of the computers in this world manipulate money, so it’s always puzzled me
that money isn’t actually a first class data type in any mainstream programming language. The
lack of a type causes problems, the most obvious surrounding currencies. If all your calculations
are done in a single currency, this isn’t a huge problem, but once you involve multiple currencies
you want to avoid adding your dollars to your yen without taking the currency differences into
account. The more subtle problem is with rounding. Monetary calculations are often rounded to the
smallest currency unit. When you do this it’s easy to lose pennies (or your local equivalent)
because of rounding errors.